I'm not talking about copyist errors. I mean conscious changes made by the scribe where either the text says one thing and is changed to say something else or where the scribe makes up an entirely new piece of content not in the original and tries to pass his change off as part of the original text.
An example of the first type would be changing "Mary" (of Bethany) to "the sisters" with the accompanying verbs scratched out and switched from singular to plural in the Book of John. I wouldn't say that is nefarious, but it is a conscious change and not a dropped letter or some other kind of goof. It's obvious that the scribe thought about it first and then changed the text.
The second type is the wholesale insertion of new content like the second ending of Mark or the Johannine Comma. Both of these are attempts at pious fraud.
Well, the comma Johanneum comes from the Latin Vulgate and was a corruption that entered the Greek manuscripts. There is nothing to suggest it was done intentionally. I am willing to give Jerome the benefit of the doubt on this.